As the scale of semiconductor integrated circuits which are typified by a DRAM becomes larger and the miniaturization of ICs progresses, the device area becomes smaller generation by generation. For a DRAM each of its memory cells is comprised of a single transistor and a single capacitor, the reduction in device area decreases the area of the capacitor that stores information, thereby lowering the information memory capability.
For semiconductor memory devices like DRAMs, various attempts have been made to secure a sufficient capacitance of each capacitor in order to prevent the information memory capability from being impaired by the large scale integration and miniaturization.
One of the attempts is to use a material which has a higher dielectric constant than the conventional oxide SiO.sub.2 /SiN film. Materials with a high dielectric constant include strontium titanate (SrTiO.sub.3) and strontium barium titanate ((Ba, Sr)TiO.sub.3) (BST).
Studies are also being made on the use of a thin film of a functional material of a ferroelectric substance such as Pb(Zr.sub.1-x, Ti.sub.x)O.sub.3 (PZT) or SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT), and people have begun to propose devices with very new functions, such as an FRAM (Ferroelectric Random Access read write Memory).
In using those dielectric substances in capacitors of semiconductor memory devices, it is typical to use noble metals like Pt, Ru and Ir or their oxides as the upper and lower electrodes that sandwich the capacitor insulating film.
The reason for the above is that BST, PZT and SBT are oxides and the lower electrode is exposed to an oxygen environment at the time of forming those films, so that it is essential to prevent oxidization of the surface of the lower electrode.
It has recently been reported that the characteristic of a capacitor is improved by using a conductive perovskite oxide film like an SrRuO.sub.3 film as the capacitor electrode. That is, the use of this type of a capacitor can dramatically improve the reliability of capacitors in a DRAM or FRAM.
In adapting a capacitor whose capacitor insulating film is formed of a high dielectric substance or ferroelectric substance in a semiconductor memory device of a large scale integration like a DRAM or FRAM, CVD is used as a scheme of forming the capacitor electrode as well as a scheme of forming the capacitor insulating film for the following reason.
Attempts to provide larger integration and further miniaturization of semiconductor memory devices like a DRAM will continue in the future and it is essential to form capacitors into a three-dimensional structure in order to secure the necessary amount of signals for the operation of such a semiconductor memory device. CVD which can be expected to provide an excellent step coverage is therefore effective in making such a design.
The formation of a conductive perovskite oxide thin film by CVD is disclosed in Jpn. Pat. Appln. KOKAI Publication No. Hei 9-27602. This method forms a (Ba.sub.1-x Sr.sub.x)RuO.sub.3 film by CVD using Ba(THD).sub.2, Sr(THD).sub.2 and Ru(Cp).sub.2 as raw materials where THD is C.sub.11 H.sub.19 O.sub.2 (2,2,6,6-tetramethyl-3,5-heptanedionato) and Cp is C.sub.5 H.sub.5.
A perovskite oxide has a conductivity only in the vicinity of the stoichiometric composition, and its electric conductivity gets lower as the composition deviates from the stoichiometric composition. As the composition of SrRuO.sub.3 deviates from Sr/Ru=1, for example, the electric conductivity will not be seen. When Sr/Ru&gt;1 at which the composition approaches SrO, particularly, this tendency becomes prominent.